In general, the feed for Claus plants consists of hydrogen sulfide-containing gases obtained during the regeneration step of gas purification processes. These gas purification processes, which are required in order to reduce the quantity of sulfur-containing components (in most cases, principally hydrogen sulfide) of industrial gases such as refinery gas, natural gas, or synthesis gas obtained by the partial combustion of hydrocarbons or coal to an acceptably low level before they are used further, generally comprise the absorption of the sulfur components from the gases into a liquid absorbent which is subsequently regenerated in order to obtain hydrogen sulfide-rich gases. The latter gases are then subsequently supplied to a Claus plant, elemental sulfur being obtained from the sulfur components. The off-gases from the Claus plant can then in turn be treated themselves also in order to reduce to a very low level the sulfur content of the gases ultimately discharged to the atmosphere.
A process for the reduction of the total sulfur content of the off-gases originating from Claus plants has already been proposed. In such a process, the off-gases are passed at a temperature higher than 175.degree. C., together with a hydrogen and/or carbon monoxide-containing gas, over a sulfided catalyst of a metal of Group VI and/or Group VIII, which catalyst is supported on an inorganic oxide carrier, after which the thus treated off-gases are passed through a liquid and regenerable absorbent for hydrogen sulfide and the nonabsorbed portion of the off-gases, optionally after combustion, are discharged to the atmosphere while the hydrogen sulfide-enriched absorbent is regenerated and reused for further absorption of hydrogen sulfide and the hydrogen sulfide-rich gas mixture liberated during the regeneration is passed to a Claus plant. Preferably, the hydrogen sulfide-rich gas mixture liberated during the regeneration is recycled to the Clause plant whence the off-gases originated so that a closed process is obtained.
In addition to hydrogen sulfide, the industrial gases often also contain other sulfur compounds, such as carbonyl sulfide, mercaptans, and disulfides, which organic sulfur compounds must likewise be removed before the gases can be used. These industrial gases, however, as a rule also contain carbon dioxide. Liquid absorbents for the removal of hydrogen sulfide generally prove to be good absorbents for carbon dioxide as well. In practice, for the removal of an organic sulfur compound such as carbonyl sulfide, a liquid absorbent is used which is in particular an efficient solvent for carbon dioxide. Consequently, after regeneration of the absorbent, a hydrogen disulfide/carbon dioxide-containing gas will be obtained which subsequently requires further processing in a Claus plant. As long as such gases have a relatively low carbon dioxide content (up to about 15 percent by volume) and a relatively high hydrogen sulfide (more than 25 percent by volume), they can be readily processed in a Claus plant. However, if the hydrogen sulfide content of the gases obtained in the regeneration of loaded liquid absorbents is less than 25 percent by volume and the carbon dioxide content of said gases is equal to or higher than the hydrogen sulfide content, the Clause plant is difficult to operate in respect to its thermal zone, and special measures have to be taken during the combustion of a portion of the hydrogen sulfide for the production of the quantity of sulfur dioxide required for the Claus reaction. These operational difficulties arise largely because the requisite temperatures for the combustion of the hydrogen sulfide cannot be attained in the thermal zone.
The present invention envisages a process for the treatment of gases which, in addition to a relatively low hydrogen sulfide content, also contain carbonyl sulfide and/or mercaptans and carbon dioxide in such a way that the gas mixture obtain after regeneration of the loaded liquid absorbent can be processed into elemental sulfur in a sulfur recovery unit. Another object of the invention is to carry out the process in such a way that the sulfur recovery efficiency of the sulfur recovery unit is increased by additionally subjecting its off-gases to a further treatment for the removal of residual sulfur compounds so that the off-gases may be discharged to the environment without objection.